Retrofocus type wide-angle photographic lens system

ABSTRACT

A compact retrofocus type wide-angle photographic lens system wherein aberrations are favorably corrected and which comprises a first lens component having negative power, a second lens component having positive power, a third negative lens component, a fourth positive meniscus lens component and a fifth positive lens component, said wide-angle photographic lens system being so adapted as to meet the following conditions. 
     
         f &lt; f.sub.B &lt; 1.2f                                         (1) 
    
     
         0.5f &lt; Σd &lt; 0.57f                                    (2) 
    
     
         0.55f &lt; f.sub.4,5 &lt; 0.65f                                  (3) 
    
     
         | r.sub.a | &lt; 1.5f                       (4)

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to a retrofocus type large aperture wide-angle photographic lens system.

(B) Description of the Prior Art

In the field of lens systems for photographic cameras, performance of retrofocus type lens systems have been remarkably improved in recent days mainly by increasing the number of lens component. However, increase in the number of lens components can not always assure enhancement in performance of lens systems but makes it impossible to design compact lens systems. Such a measure for improvement of lens systems is therefore very disadvantageous in manufacturing photographic lens systems.

SUMMARY OF THE INVENTION

A general object of the present invention is to provide a very compact retrofocus type wide-angle large aperture lens system comprising a small number of lens components wherein aberrations are corrected favourably.

The photograhic lens system according to the present invention comprises only five lens components wherein the first component is a meniscus lens component having negative power, the second component is a lens component having positive power, the third component is a negative lens, the fourth component is a positive meniscus lens and the fifth component is a positive lens. The lens systems according to the present invention satisfies the following conditions:

    f < f.sub.B < 1.2f                                         (1)

    0.5f < Σd < 0.57f                                    (2)

    0.55f < f.sub.4,5 < 0.65f                                  (3)

    | r.sub.a | > 1.5f                       (4)

wherein the reference symbols represent as defined below:

F: TOTAL FOCAL LENGTH OF THE ENTIRE LENS SYSTEM

F_(B) : back focal length

F₄,5 : TOTAL FOCAL LENGTH OF THE FOURTH AND FIFTH LENS COMPONENTS CONSIDERED AS A WHOLE

ΣD: DISTANCE AS MEASURED FROM THE FRONT SIDE OF THE SECOND LENS COMPONENT TO THE REAR SIDE OF THE THIRD LENS COMPONENT

R_(A) : RADIUS OF CURVATURE OF THE REAR SURFACE OF THE SECOND LENS COMPONENT

When f_(B) exceeds the upper limit of condition (1) (f_(B) > 1.2f) in the lens system described above, f has a small value and the field angle becomes large since f_(B) has a nearly constant value in lens systems adapted for use with a single-lens reflex cameras. It is therefore impossible to favourably correct aberrations in a lens system comprising a small number of lens components and having a large aperture like the lens system according to the present invention. The condition of f_(B) < 1.2f is required for this reason. If f_(B) has a value smaller than the lower limit of condition (1) (f > f_(B)), on the other hand, f_(B) has a small value and it becomes impossible to design a retrofocus type wide-angle lens system adapted for use with a single-lens reflex camera.

In the second place, condition (2) is necessary for eliminating curvature of field and astigmatism since it is impossible to sufficiently correct these aberrations when Σd is smaller than 0.5f. If Σd has too small a value, it is impossible to select a small diameter for the front lens since a large airspace is required between the first and second lens components. The entire lens system becomes unavoidably large when Σd has a value smaller than the lower limit of condition (2). When Σd has a value larger than 0.57f, on the other hand, the above-mentioned aberrations will be overcorrected and it is made impossible to properly correct the aberrations in the entire lens system.

Condition (3) is effective for favourably correcting spherical aberration and coma. When f₄,5 is smaller than 0.55f, spherical aberration is aggravated. When f₄,5 is larger than 0.65f, on the other hand, coma is overcorrected especially for upper rays and, further, the rear lens group with positive power has a longer focal distance as a whole, thereby shifting forward the conjugate position corresponding to the plane of the image formed by the rear lens group. The total length of the entire lens system is therefore prolonged and it becomes impossible to accomplish the object of the present invention to provide a compact lens system.

Condition (4), which can exhibit sufficient effect in combination with the other conditions, is required for favourably correcting coma. When this condition is not met, coma can not be favourably corrected for lower rays. Speaking more concretely, coma is overcorrected when r_(a) has a positive value and undercorrected when it has a negative value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view illustrating the composition of a preferred embodiment of the present invention;

FIG. 2A through FIG. 2C illustrate curves showing the aberration characteristic of the embodiment 1;

FIG. 3 shows a sectional view illustrating the composition of another embodiment of the present invention;

FIG. 4A through FIG. 4C show graphs illustrating the aberration characteristic of the embodiment 2;

FIG. 5 illustrates a sectional view showing the composition of a third embodiment of the present invention; and

FIGS. 6A through 6C show graphs illustrating the aberration characteristic of the embodiment 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, some numerical data of the lens system according to the present invention will be exemplified below:

    ______________________________________                                         Embodiment 1                                                                   ______________________________________                                         r.sub.1 = 0.89501                                                               d.sub.1 = 0.19296                                                                            n.sub.1 = 1.72                                                                               ν.sub.1 = 42.1                                 r.sub.2 = 12.63956                                                              d.sub.2 = 0.05513                                                                            n.sub.2 = 1.51633                                                                            ν.sub.2 = 64.2                                 r.sub.3 = 0.38615                                                               d.sub.3 = 0.40384                                                             r.sub.4 = 1.17183                                                               d.sub.4 = 0.15409                                                                            n.sub.3 = 1.8061                                                                             ν.sub.3 = 40.9                                 r.sub.5 = -0.55000                                                              d.sub.5 = 0.05513                                                                            n.sub.4 = 1.69895                                                                            ν.sub.4 = 30.1                                 r.sub.6 = 17.11794                                                              d.sub.6 = 0.09648                                                             r.sub.7 = -0.81932                                                              d.sub.7 = 0.23073                                                                            n.sub.5 = 1.80518                                                                            ν.sub.5 = 25.4                                 r.sub.8 = 2.53199                                                               d.sub.8 = 0.03584                                                             r.sub.9 = -3.16141                                                              d.sub.9 = 0.12818                                                                            n.sub.6 =  1.757                                                                             ν.sub.6 = 47.9                                 r.sub.10 = -0.59818                                                             d.sub.10 = 0.00276                                                            r.sub.11 = 1.77635                                                              d.sub.11 = 0.08270                                                                           n.sub.7 = 1.757                                                                              ν.sub.7 = 47.9                                 r.sub.12 = -5.55394                                                            f = 1.0 , f.sub.B = 1.06413 , F/2.0                                            f.sub.4,5 = 0.621 , Σd = 0.53643                                         ______________________________________                                    

    ______________________________________                                         Embodiment 2                                                                   ______________________________________                                         r.sub.1 = 0.92105                                                               d.sub.1 = 0.19288                                                                            n.sub.1 = 1.72342                                                                            ν.sub.1 = 38.0                                 r.sub.2 = 5.22870                                                               d.sub.2 = 0.05511                                                                            n.sub.2 = 1.51633                                                                            ν.sub.2 = 64.2                                 r.sub.3 = 0.37584                                                               d.sub.3 = 0.41551                                                             r.sub.4 = 1.25533                                                               d.sub.4 = 0.15816                                                                            n.sub.3 = 1.8061                                                                             ν.sub.3 = 40.9                                 r.sub.5 = -0.46704                                                              d.sub.5 = 0.05511                                                                            n.sub.4 = 1.69895                                                                            ν.sub.4 = 30.1                                 r.sub.6 = -20.22105                                                             d.sub.6 = 0.12399                                                             r.sub.7 = -0.83307                                                              d.sub.7 = 0.19233                                                                            n.sub.5 = 1.80518                                                                            ν.sub.5 = 25.4                                 r.sub.8 = 2.48493                                                               d.sub.8 = 0.04602                                                             r.sub.9 = -2.94809                                                              d.sub.9 = 0.09258                                                                            n.sub.6  = 1.757                                                                             ν.sub.6 = 47.9                                 r.sub.10 = -0.59996                                                             d.sub.10 = 0.00276                                                            r.sub.11 = 2.14910                                                              d.sub.11 = 0.08266                                                                           n.sub.7 = 1.762                                                                              ν.sub.7 = 40.2                                 r.sub.12 = -3.42802                                                            f = 1.0 , f.sub.B = 1.08251 , F/2.0                                            f.sub.4,5 = 0.629 , Σd = 0.52959                                         ______________________________________                                    

    ______________________________________                                         Embodiment 3                                                                   ______________________________________                                         r.sub.1 = 1.52399                                                               d.sub.1 = 0.04666                                                                            n.sub.1 = 1.51633                                                                            ν.sub.1 = 64.2                                 r.sub.2 = 0.44075                                                               d.sub.2 = 0.33470                                                             r.sub.3 = 0.65227                                                               d.sub.3 = 0.36655                                                                            n.sub.2 = 1.72                                                                               ν.sub.2 = 50.3                                 r.sub.4 = -1.64457                                                              d.sub.4 = 0.12852                                                             r.sub.5 = -0.54644                                                              d.sub.5 = 0.04414                                                                            n.sub.3 = 1.74077                                                                            ν.sub.3 = 27.8                                 r.sub.6 = 1.36494                                                               d.sub.6 = 0.03911                                                             r.sub.7 = -0.89578                                                              d.sub.7 = 0.06817                                                                            n.sub.4 = 1.757                                                                              ν.sub.4 = 47.9                                 r.sub.8 = -0.42798                                                              d.sub.8 = 0.00279                                                             r.sub.9 = 1.66768                                                               d.sub.9 = 0.06733                                                                            n.sub.5 = 1.713                                                                              ν.sub.5 = 53.9                                 r.sub.10 =  -2.76854                                                           f = 1.0 , f.sub.B = 1.08095 , F/2.8                                            f.sub.4,5 = 0.598 , Σd = 0.53921                                         ______________________________________                                    

Of the embodiments described above, the embodiment 1 is illustrated in FIG. 1 in which reference symbol Σd represents d₄ + d₅ + d₆ + d₇ and reference symbol r_(a) denotes r₆. Similarly in the embodiment 2 which is illustrated in FIG. 3, reference symbol Σd represents d₄ + d₅ + d₆ + d₇ and reference symbol r_(a) denotes r₆. In the embodiment 3 which is illustrated in FIG. 5, wherein the first and second lens components are composed of a single lens element respectively unlike the two embodiments mentioned above, reference symbol Σd represents d₃ + d₄ + d₅ and reference symbol r_(a) denotes r₄. 

I claim:
 1. A retrofocus type wide-angle photographic lens system comprising a first negative cemented doublet lens component, a second positive cemented doublet lens component, a third negative lens component, a fourth positive meniscus lens component and a fifth positive lens component, and said wide-angle photographic lens system having the following numerical data:

    ______________________________________                                         r.sub.1 = 0.89501                                                               d.sub.1 = 0.19296                                                                            n.sub.1 = 1.72                                                                               ν.sub.1 = 42.1                                 r.sub.2 = 12.63956                                                              d.sub.2 = 0.05513                                                                            n.sub.2 = 1.51633                                                                            ν.sub.2 = 64.2                                 r.sub.3 = 0.38615                                                               d.sub.3 = 0.40384                                                             r.sub.4 = 1.17183                                                               d.sub.4 = 0.15409                                                                            n.sub.3 = 1.8061                                                                             ν.sub.3 = 40.9                                 r.sub.5 = 0.55000                                                               d.sub.5 = 0.05513                                                                            n.sub.4 = 1.69895                                                                            ν.sub.4 = 30.1                                 r.sub.6 = 17.11794                                                              d.sub.6 = 0.09648                                                             r.sub.7 = -0.81932                                                              d.sub.7 = 0.23073                                                                            n.sub.5 = 1.80518                                                                            ν.sub.5 = 25.4                                 r.sub.8 = 2.53199                                                               d.sub.8 = 0.03584                                                             r.sub.9 = -3.16141                                                              d.sub.9 = 0.12818                                                                            n.sub.6 = 1.757                                                                              ν.sub.6 = 47.9                                 r.sub.10 = -0.59818                                                             d.sub.10 = 0.00276                                                            r.sub.11 = 1.77635                                                              d.sub.11 = 0.08270                                                                           n.sub.7 = 1.757                                                                              ν.sub.7 = 47.9                                 r.sub.12 = -5.55394                                                            f = 1.0 ,  f.sub.B = 1.06413 , F/2.0                                           f.sub.4,5 = 0.621 ,  Σd = 0.53643                                        ______________________________________                                    


2. A retrofocus type wide-angle photographic lens system comprising a first negative cemented doublet lens component, a second positive cemented doublet lens component, a third negative lens component, a fourth positive meniscus lens component and a fifth positive lens component, and said wide-angle photographic lens system having the following numerical data:

    ______________________________________                                         r.sub.1 = 0.92105                                                               d.sub.1 = 0.19288                                                                            n.sub.1 = 1.72342                                                                            ν.sub.1 = 38.0                                 r.sub.2 = 5.22870                                                               d.sub.2 = 0.05511                                                                            n.sub.2 = 1.51633                                                                            ν.sub.2 = 64.2                                 r.sub.3 = 0.37584                                                               d.sub.3 = 0.41551                                                             r.sub.4 = 1.25533                                                               d.sub.4 = 0.15816                                                                            n.sub.3 = 1.8061                                                                             ν.sub.3 = 40.9                                 r.sub.5 = -0.46704                                                              d.sub.5 = 0.05511                                                                            n.sub.4 = 1.69895                                                                            ν.sub.4 = 30.1                                 r.sub.6 = -20.22105                                                             d.sub.6 = 0.12399                                                             r.sub.7 = -0.83307                                                              d.sub.7 = 0.19233                                                                            n.sub.5 = 1.80518                                                                            ν.sub.5 = 25.4                                 r.sub.8 = 2.48493                                                               d.sub.8 = 0.04602                                                             r.sub.9 = -2.94809                                                              d.sub.9 = 0.09258                                                                            n.sub.6 = 1.757                                                                              ν.sub.6 = 47.9                                 r.sub.10 = -0.59996                                                             d.sub.10 = 0.00276                                                            r.sub.11 = 2.14910                                                              d.sub.11 = 0.08266                                                                           n.sub.7 = 1.762                                                                              ν.sub.7 = 40.2                                 r.sub.12 = -3.42802                                                            f = 1.0 , f.sub.B = 1.08251 , F/2.0                                            f.sub.4,5 = 0.629 ,  Σd = 0.52959                                        ______________________________________                                    


3. A retrofocus type wide-angle photographic lens system comprising a first negative meniscus lens component, a second positive lens component, a third negative lens component, a fourth positive meniscus lens component and fifth positive lens component, and said wide-angle photograhic lens system having the following numerical data:

    ______________________________________                                         r.sub.1 = 1.52399                                                               d.sub.1 = 0.04666                                                                            n.sub.1 = 1.51633                                                                            ν.sub.1 = 64.2                                 r.sub.2 = 0.44075                                                               d.sub.2 = 0.33470                                                             r.sub.3 = 0.65227                                                               d.sub.3 = 0.36655                                                                            n.sub.2 = 1.72                                                                               ν.sub.2 = 50.3                                 r.sub.4 = -1.64457                                                              d.sub.4 = 0.12852                                                             r.sub.5 = -0.54644                                                              d.sub.5 = 0.04414                                                                            n.sub.3 = 1.74077                                                                            ν.sub.3 = 27.8                                 r.sub.6 = 1.36494                                                               d.sub.6 = 0.03911                                                             r.sub.7 = -0.89578                                                              d.sub.7 = 0.06817                                                                            n.sub.4 = 1.757                                                                              ν.sub.4 = 47.9                                 r.sub.8 = -0.42798                                                              d.sub.8 = 0.00279                                                             r.sub.9 = 1.66768                                                               d.sub.9 = 0.06733                                                                            n.sub.5 = 1.713                                                                              ν.sub.5 = 53.9                                 r.sub.10 = -2.76854                                                            f = 1.0 , f.sub.B = 1.08095 , F/2.8                                            f.sub.4,5 = 0.598 ,  Σd = 0.53921                                        ______________________________________                                     